Racetrack memory is a memory-storage device in which data are stored in magnetic nanowires in the form of magnetic domain walls that separate magnetic regions magnetized in opposite directions (see, for example, U.S. Pat. Nos. 6,834,005, 6,920,062, and 7,551,469 to Parkin). A key principle underlying this memory is the controlled motion of a series of such domain walls backwards and forwards along the nanowires (also known as racetracks) using nanosecond long pulses of current applied along the nanowire. Devices to inject domain walls and to detect domain walls are integrated into each of the nanowires. The domain walls are moved to the injection and detection devices by means of current pulses of the necessary length and number. The racetracks can be formed from two distinct classes of magnetic materials in which the magnetization of the material is (a) predominantly oriented within the plane and along the length of the nanowire and (b) predominantly oriented perpendicular to the length of and perpendicular to the plane of the nanowire. Materials that form class (a) are typically composed of soft magnetic materials in which the intrinsic magnetocrystalline anisotropy of the material is small compared to the shape magnetic anisotropy derived from magnetostatic energies associated with the cross-sectional shape and size compared to the length of the nanowire. In these materials the domain walls are typically wide: for example, the domain walls in nanowires formed from permalloy, an alloy of Ni and Fe in the approximate atomic composition ratio 80:20, are typically 100-200 nm wide, and these domain walls can be readily deformed. Materials that form class (b) are typically composed of ultrathin magnetic layers in which their interfaces with non-magnetic layers give rise to interfacial magnetic anisotropies that can result in their magnetization preferring to be oriented perpendicular to these interfaces. Typical examples include an ultrathin layer of Co placed adjacent to a Pt layer and multilayered structures formed from alternating layers of atomically thin Co and Pt layers. Another example are multilayers formed from ultrathin layers of Co and Ni. For such materials the width of the domain walls are smaller, the greater is the perpendicular magnetic anisotropy (PMA) and can be as narrow as 1-10 nm. Thus materials of class (b) are preferred for the fabrication of dense racetrack memories.
In prior art devices the domain walls are shifted to and fro along racetracks by current pulses in which the current is spin-polarized as a result of spin-dependent scattering within the bulk of the magnetic materials from which the racetrack is formed. The transfer of spin angular momentum from the spin polarized current to the domain wall gives rise to a torque on the magnetic moments within the domain wall that results in motion of the domain wall along the nanowire. This phenomenon of spin transfer torque (STT) results in the domain walls being driven in the direction of the flow of spin angular momentum such that spin angular momentum is transferred from the current to the magnetic moments. It is well established that in permalloy the conduction electrons that carry the electrical current are majority spin polarized, i.e., the conduction electrons have their magnetic moments oriented parallel to the direction of the local magnetic moments on the Ni and Fe atoms. This results in magnetic domain walls in permalloy nanowires moving in the direction of the flow of the conduction electrons, i.e., opposite to the direction of the electrical current. The velocity of the domain walls depends on the magnitude of the electrical current and for current densities of ˜108 A/cm2 in permalloy, the domain walls move with velocities of ˜100 m/sec.
Domain walls can be pinned by defects arising from roughness of the edges or surfaces of the nanowires. In permalloy and other materials in class (a) the interaction of the spin polarized current and the domain wall's magnetization is such that very large current densities are required to move domain walls that are pinned by even comparatively small pinning potentials. For example a current density of ˜108 A/cm2 can overcome effective pinning fields of just a few Oersted. By contrast the much narrower domain walls in materials of class (b) changes the details of the interaction of spin polarized current and the domain wall's magnetization so that much larger pinning fields can be overcome compared to the domain walls in materials of class (a) for otherwise the same current density. Since nanowires will inevitably have rough edges and surfaces this is a significant advantage of materials in class (b).
Finally, a third advantage of materials of class (b) is that racetracks with PMA can be made magnetically very thin, just a few atomic layers thick, and yet the domain walls can be stable against thermal fluctuations because of the very large PMA. Since the magnetic nanowires are very thin, and therefore contain proportionally less magnetic moment, domain walls can be injected into the nanowires using injection devices that use spin torque transfer from currents injected across tunnel barriers into the racetracks. For materials in class (a) the racetracks of prior art devices have to be formed from much thicker magnetic layers in order to stabilize domain walls with a vortex domain structure that can be moved with currents. In thinner racetracks formed from materials of class (a) the domain walls have a transverse wall structure that requires much higher current densities to move them.